Pulverized coal burners are well known. For example, U.S. Pat. No. 4,147,116 describes a swirl burner in which pulverized coal is delivered with a spiraling motion to centrifugally concentrate at the perimeter of a central tube. The pulverized coal is then diffused into a conical pattern by a vaned diffuser located at the discharge end of the central tube. This type of burner is essentially a swirl burner with which a large bulky flame is produced whose shape and travel may be somewhat controlled by adjusting the position of a deflector.
U.S. Pat. No. 4,434,727 describes a pulverized coal burner wherein combustion air surrounds a central coal delivery tube whose discharge end is shaped to widely disperse the fuel. The nozzle presents a significant blockage to the fuel flow, thus causing a significant pressure drop and load on the forced draft fan. This burner also tends to create a central region of low oxygen content that is difficult to ignite.
U.S. Pat. No. 4,422,389 illustrates a pulverized coal burner wherein the pulverized coal fuel is delivered through an annular conduit that surrounds a central conduit through which combustion air is supplied. U.S. Pat. No. 4,350,103 describes a solid fuel burner in which a central stream of pulverized coal is passed through a premix chamber and is shrouded by a secondary gas stream.
A swirl-type burner for solid fuel formed of pulverized coal is shown in U.S. Pat. No. 4,515,090. This burner produces a turbulent flame intended to enhance the mixing of primary and secondary air with the pulverized fuel. Another swirl type pulverized coal burner is shown in U.S. Pat. No. 4,457,241.
In an electric utility station, an array of burners is commonly used to provide the required boiler heat. It is often necessary to vary the heat output from the array of burners to match partial electrical loads. This can be done in part by reducing, i.e., turning-down, the amount of heat produced by each burner. Such turn-down can be obtained by reducing the combustion air and fuel through the burner. When the turn-down ratio, however, becomes too high, the swirl effect tends to reduce to a level where combustion may not be sustained or the flame temperature changes so as to increase pollutant problems such as excessive production of SO.sub.3. Hence, in practice, when substantial heat reduction is required, individual burner turn-down is but partially relied upon and selected burners in the array are actually shut-down. Such complete turn-off of burners, however, affects the temperature profile of the array and may create undesirable cold spots.
The swirl type solid fuel burners produce a large bulky flame that is stabilized in a region in front of the burner by the swirling motion. The swirl burners produce strong turbulent flames whose interactions with each other in an array is difficult to control and predict so that interference between the flames can be detrimental. The use of a strongly turbulent high fuel velocity swirling burner results in high coal particles velocities relative to air to promote combustion. It is very difficult to model the airflow to achieve uniform distribution of combustion air in a multiburner or array type installation.
Such high coal particle velocities is generally recommended to promote the supply of oxygen to burning coal particles and thus reduce the effect from a normally slower process whereby oxygen diffuses to the shrinking burning particle. See for example, articles entitled "Burning Characteristics of Pulverized Coal" by A. B. Hedley and M. E. Leesley, published during 1965 in the Journal of the Institute of Fuels; "Conditions For The Stable Burning Of Carbon In An Airstream" by D. B. Spalding at pages 288-295 of the Journal of the Institute of Fuels published Dec., 1953; and "Combustion Of Millimeter Sized Coal Particles In Convection Flow" by K. W. Ragland and J. T. Yong published in Combustion And Flame at pages 285-297 in 1985.
Natural gas burners are known, such as the DYNASWIRL venturi register gas burner, manufactured and sold by the assignee of this invention, and with which an elongated flame is produced with both an axial gasflow and a vortex to stabilize the flame. The gaseous fuel is supplied through a plurality of tubes whose end faces are oriented to direct the fuel through specially drilled injector faces towards the primary air flow stream. The tubes are also tangentially oriented.